This invention relates to a welding method of aluminum alloys to prevent a formation and contamination of black or dark gray or brown particulate substances (hereafter called as a black powder) around or on the weldment bead during an electrode-type inert gas arc welding (hereafter called as a MIG welding) of aluminum alloys.
Demands for aluminum alloys are ever-growing in various industrial sectors including the aerospace motor-vehicles, chemical vessels and containers, constructions, electric machineries and other industries due to their light weight, excellent corrosion resistance, high heat and electric conductivities, and high specific strength (that is, high ratio of strength-to-weight).
Numerous aluminum alloy components utilized and constructed in the aforementioned industries are assembled by either welding, soldering or mechanical assembling. By welding of these components, the MIG welding or non-electrode-type inert gas arc welding (hereafter cited as a TIG welding) are principally employed. Since MIG welding shows relatively higher arc efficiency than the TIG welding, and is generally characterized by high welding speeds, it is utilized for welding plate/sheet materials in a wide range of plate thicknesses.
Where using the MIG technique to weld aluminum alloys, it is occasionally noticed that black powder is formed around the weldment bead or on the bead surface itself, depending upon the type of base material being welded, the welding wire, being used and other welding conditions. Although the black powder is not, in general, considered a weld defect such as weld cracks or pits, the black powder normally spoils the appearance of weldments.
Hence, care is usually taken to minimize the formation and contamination of the black powder by an appropriate selection of various welding conditions including torch angle, arc voltage, the shape and inner diameter of the shield nozzle, the distance between the nozzle base portion and the base material and the flow rate of the shield gas.
The black powder formed during the MIG welding of aluminum alloys can also cause problems when post-welding treatment are needed such as a surface treatment or painting. Therefore, prior to these post-welding treatments, the black powder is removed by mechanical means such as by brushing, or sometimes by chemical or other physical methods.
Black powder seems to form easily and in greater amount when a Mg-bearing (as a main alloying element) 5000-series aluminum alloy or a Zn-, Mg-bearing (as main alloying elements) 7000-series aluminum alloy according to JIS H 4000 "Aluminum and Aluminum Alloy Plate and Bar" is welded by using alloy wire made of 5000-series aluminum alloy according to JIS Z 3232 "Aluminum and Aluminum Alloy Welding Bar and Wire."
Although black powder appears to be very fine particles by a macroscopic observation, a microscopic observation shows that it is composed of very-fine spherical particles consisting of nearly pure aluminum, and alloying elements such as Mg, Zn, Si (depending upon the type of welding wire) and Oxygen deposited over pure aluminum spherical particles.
It is recognized that the amount of the black powder deposited is removed by the following welding conditions; namely, by an increased angle welding, a low arc voltage, a large inner diameter of the shield nozzle, and a short distance between the nozzle base portion and the base material to be welded.